The present invention relates to the structure of a coil bobbin and, in particular, relates to a dual coil bobbin consisting of two overlapped coil bobbins, or an outer coil bobbin and an inner coil bobbin. The present coil bobbin is utilized for a transformer in a switching power supply circuit.
A conventional dual coil bobbin is shown in FIGS. 1 through 3. FIG. 1 is a disassembled perspective view of the conventional dual coil bobbin, FIG. 2 is a cross sectional view of a transformer assembly utilizing the coil bobbin of FIG. 1, and FIG. 3 is a cross sectional view along the line A.sub.1 --A.sub.1 of FIG. 2. In those figures, the dual coil bobbin is assembled by an outer bobbin 1 and an inner bobbin 2, both of which are made of dielectric material such as synthetic resin. The outer bobbin 1 and the inner bobbin 2 have a cylindrical hollow members 12 and 21, respectively. The radius of the member 12 is greater than that of the member 21.
The cylindrical member 12 of the outer bobbin 1 has two flanges 13 and 14 mounted around both the ends thereof. The flange 13 has a terminal plate 15 having a plurality of terminal pins 16 for the connection of lead wires 42 of a coil 3 fitted on the cylindrical member 12 to an external circuit (not shown). The terminal plate 15 has also a plurality of grooves 26 which are formed on its top surface. Those grooves 26 accommodate the lead wires 42 extended from the coil 3 to the terminal pins 16. The cylindrical member 21 of the inner bobbin 2 has a flange 22 mounted around one end thereof. The flange 22 has terminal plates 23 with a plurality of terminal pins 24 and grooves 25 which are the same configuration as those of the outer bobbin 1. The terminal pins 24 is to connect lead wires 41 of a coil 4 fitted on the cylindrical member 21 to an external circuit. The grooves 25 accommodate the lead wires 41 extended from the coil 4 to the terminal pins 24.
When the transformer is assembled by the outer bobbin 1 with the coil 3 and the inner bobbin 2 with the coil 4, the cylindrical member 21 of the inner bobbin 2 is inserted into the axial bore 11 of the cylindrical member 12 of the outer bobbin 1 so that the termianl plates 15 and 23 are located in parallel with each other in the horizontal direction, as shown in FIG. 2. Of course, the transformer assembly has a cylindrical core 5 made of magnetic material inserted into the axial bore of the cylindrical member 21. The transformer utilizing the dual coil bobbin of FIG. 1 is mounted on a printed circuit board (not shown) by using terminal pins 25 and 26. In this case, the coil 3 of the outer bobbin 1 is used as a secondary winding applied with a low voltage and the coil 4 of the inner bobbin 2 is used as a primary winding applied with a high voltage.
In this case, it should be noted that the coil bobbin utilized for a power transformer must satisfy the lawful safety standards issued in each country. It is known that a creeping insulation distance is the most important factor regulated by safety standards. The creeping distance stands for the shortest distance between adjacent conductors measured along the surface of a solid dielectric material interposed therebetween. If the creeping distance between adjacent conductors interposed by solid dielectric material is short, insulation between those conductors would not be established sufficiently. Therefore, the spark discharge would sometimes arise between them. Therefore, the creeping distance sufficient for establishing insulation is regulated by the safety standards in view of safety in each country.
In the above mentioned dual coil bobbin, the grooves 25 are provided in order to elongate the creeping distance between two adjacent lead wires 41--41 accommodated therein to make sure of insulation therebetween. The presence of the grooves 25 of the inner bobbin 2 is very effective because the lead wires 41 accommodated in the grooves 25 handle the high voltage. The grooves 26 of the outer bobbin 1 have the same function as the grooves 25.
On the other hand, insulation between the coil 3 and the lead wires 41 of the coil 4 must be established as well. The creeping distance between those conductors must extend about 6 millimeters according to the safety standard issued in each country. The creeping distance between the coil 3 and the lead wires 41 is defined by the flange 14. In this case, the creeping distance between the coil 3 and the lead wire 41-1 passing through the groove 25-1 located in the middle of the terminal plate 23 differs from that between the coil 3 and the lead wire 41-2 passing through the groove 25-2 located in the extreme end of the plate 23. First, as shown in FIG. 2 the opening end of the groove 25-1 on the inside face of the plate 23 is substantially covered by the flange 14. Therefore, the creeping distance between the coil 3 and the lead wire 41-1 accommodated in the groove 25-1 is relatively long, so that insulation between those conductors is assured to some extent. On the other hand, as shown in FIG. 3, the opening end of the groove 25-2 on the inside face of the plate 23 is not covered by the flange 14 at all. This means the creeping distance between the coil 3 and the lead wire 41-2 accommodated in the groove 25-2 is very short. Therefore, it is impossible to establish the creeping distance of 6 millimeters regulated by the safety standards.
In order to overcome this disadvantage, and elongate the creeping distance between the coil 3 and lead wires 41 accommodated in the grooves 25, as shown in FIG. 4, insulating tapes 6 and 7 are provided around both the ends of the cylindrical member 12, or as shown in FIG. 5 the coil 3 is partially wrapped by an insulating tape 8. However, the presence of the insulating tapes 6 and 7 causes the cross sectional area to reduce to deteriorate performance of the transformer. On the other hand, use of the insulating tape 8 causes the assembly operation to be complicated and prevents application of automatic assembly operation.